The present invention relates to an air fuel ratio control system for a two-cycle engine wherein an intake air quantity is estimated by a throttle opening degree, and a basic fuel injection quantity is set by the estimated intake air quantity.
Recently, two-cycle engines including the following structure have been proposed. The engines use an injector to improve the response of an engine speed not only within a high speed range but also within a low speed range, and to purify exhaust gas emission.
For example, Japanese Utility Model Laid-open No. 58-169117(1983) discloses an air-fuel ratio control system for a two-cycle engine. In the system, a fuel injection quantity is set by an intake air quantity and an engine speed as parameters, and the fuel is injected from the injector at the predetermined injection timing.
Generally, there are two types of intake air quantity measurement systems for engines. One is measuring the intake air quantity with an intake air quantity sensor as in the Publication. The other estimates an intake air quantity from the engine speed and a throttle opening degree. The latter estimating type has simple structure and low production costs, so that it is used mainly for two-cycle engines.
In the estimating type, the intake air quantity has a complicated function relative to the engine speed and the throttle opening degree. It is therefore difficult in practice to estimate the intake air quantity correctly. Namely, the air density changes with the temperature of an intake air and with the temperature condition of the engine, even though the system has a constant engine speed and a constant throttle opening degree, thereby varying the charging efficiency to a large extent.
Accordingly, a proper air-fuel ratio of the engine has been obtained in the estimating type by correcting the estimated intake air quantity in dependency on various increment correction coefficients. These coefficients are set in accordance with an actual intake air temperature and coolant temperature of the engine under operation.
However, in case of two-cycle engine, the intake air is not directly supplied to a combustion chamber in difference with a four-cycle engine. In a two-cycle engine, the intake air is once supplied to a crank chamber also serving to a pressure chamber via a scavenging air passage under the pressure within the crank chamber exerted upon a down stroke of a piston during an ignition expansion cycle. Therefore, the intake air of the two-cycle engine remains within the engine longer than in a four-cycle engine, so that the temperature condition of the crank case gives a great influence on the air density required at the time of setting an air-fuel ratio.
Accordingly, in the conventional system not taking the temperature of the crank case into consideration, the fuel injection quantity is not set properly even with aforementioned various correction coefficients, thereby posing the problems of a poor controllability of the air-fuel ratio, and hence lowering the engine output and contaminating the exhaust gas emission.